1. Technical Field
The present invention relates to electrodes that are useful for medical applications. More particularly, the present invention relates to forming an electrically and mechanically reliable connection to a cardiac electrical stimulation and/or sensing electrode.
2. Description of the Prior Art
The use of electrical signals to stimulate or steady heart rhythm (pacing) or to restore heart rhythm when the muscle fibers of the heart undergo very rapid irregular contractions, which result in a lack of synchronism between the heartbeat and the pulse (defibrillation), is a well accepted, lifesaving medical technique. Pacing and defibrillation devices that produce these electrical signals typically consist of an electrical signal generator that is implanted inside a patient's body. The electrical signals produced by the signal generator are coupled to the patient's heart with an electrical lead that includes a conductor portion for carrying the electrical signal from the signal generator to the patient's heart and an electrode portion that makes actual electrical contact with the patient's heart.
Leads for pacing and defibrillation are available for implantation around the heart, i.e. an epicardial lead. When such leads are used for pacing they are generally placed around the right ventricle or right atrium of the patient's heart, and when used for defibrillation they are generally placed around the right or left ventricle of the patient's heart. FIG. 1 shows a typical epicardial lead 10, in which a conductor 11 is joined to an electrode 12 at a joint 13 by crimping and/or welding onto a sleeve 15. Examples of epicardial leads are shown in U.S. Pat. Nos. 4,314,095, Device and Method for Making Electrical Contact, and 4,827,932, Implantable Defibrillation Electrodes.
Endocardial electrical stimulation leads are available for pervenous insertion to introduce the lead into the right ventricle or right atrium of the patient's heart for pacing, and into the patient's right ventricle or superior vena cava, which is slightly above the heart, for defibrillation (see FIGS. 2a and 2b, discussed below). Examples of endocardial leads are shown in U.S. Pat. Nos. 4,679,572, Low Threshold Cardiac Pacing Electrodes, 4,662,382, Pacemaker Lead with Enhanced Sensitivity, 4,458,695, Multipolar Electrode Assembly for Pacing Lead, 4,161,952, Wound Wire Catheter Cardioverting Electrode, and 4,844,099/4,784,161, Porous Pacemaker Electrode Tip Using a Porous Substrate.
Surgical procedures for implanting electrical leads into a patient's heart should be minimally intrusive. For this reason, endocardial leads, which are introduced into a patient's vein by means of a hollow tubular introducer, and anchored in the right ventricle by a fixation mechanism, or placed in the superior vena cava, coronary sinus, etc., are preferred over epicardial leads, which must be surgically implanted by opening the patient's chest. Although endocardial leads are less intrusive than epicardial leads, endocardial leads must pass through a vein and/or heart valve. It is therefore important to provide a lead with as small a diameter as possible, and thereby limit or eliminate entirely any damage to fragile veins or impairment of valve function that may be caused by the lead. It is also critical to provide a highly flexible lead, both for ease of positioning and for long term behavior of the lead within a patient's beating heart. Increased flexibility increases the fatigue life of the lead in the heart and thus reduces the likelihood of a lead failure.
In an endocardial lead 16, such as that shown in FIG. 2a, the conductor 17 and the electrode 18 are coupled at a joint 19 that must provide a low resistance electrical connection and a secure mechanical connection, while exhibiting a narrow profile to minimize intrusion of the lead into the patient's vein. Common practice is to mechanically join the conductor to the electrode at a sleeve portion 20 using a crimp or swage; and/or to form the joint with heat by brazing or welding.
In FIG. 2b, an electrical stimulation lead 22 is shown having a coiled conductor 23 that is joined to a sleeve 24 by passing a portion of the distal end of the conductor through an aperture 26 formed in the side of the sleeve 24. The joint 25 between the conductor and the sleeve is completed by welding the distal end of the conductor to the outer sleeve surface. An electrode (not shown) may be joined to the sleeve surface as appropriate.
The joints shown in FIGS. 2a and 2b are not optimal for endocardial applications. In the joint shown in FIG. 2a, the sleeve must be both thick enough to provide sufficient strength to maintain a crimp bond, and the sleeve must have sufficient length to secure the crimp; while in the joint shown in FIG. 2b, the protrusion of the distal conductor end through the side of the electrode significantly increases the thickness of the electrode. The flexibility of each of these joints is reduced by the increased length needed when a multifilar coil is used as the conductor because a crimp or a weld must attach all of the wires of the coil in a row, thereby adding the diameter of the wire for each wire in the coil to the length of the joint.
Examples of various other joints having application in medical electrical stimulation electrodes are shown in U.S. Pat. Nos. 4,161,952, Wound Wire Catheter Cardioverting Electrode, 4,214,804, Press Fit Electrical Connection Apparatus, and 4,328,812, Ring Electrode for Pacing Lead.
Electrical joints in chronically implanted endocardial leads are subject to cyclical stresses, and should therefore be mechanically stable with respect to fatigue behavior. That is, due to the critical nature of the application to which the leads are put, a joint failure can have catastrophic results. Because the lead is implanted into a human body, it must also be both biocompatible (i.e. non-toxic) and corrosion resistant. The best conductors, for example silver and copper, are both toxic and subject to corrosion within the human body.
It would therefore be highly desirable to provide a lead for medical electrical stimulation applications having low electrical resistance, high mechanical and fatigue strength, biocompatibility, and high corrosion resistance, while minimizing intrusiveness of the lead into the patient's body, minimizing lead thickness, and maximizing flexibility.